1. Field of the Invention
The present invention relates to a neutral atom trapping device, and in particular to a neutral atom trapping device which specializes a multipole-magnetic field-generating electrode in a magneto-optical trap or/and a magnetic trap to enhance a magnetic quadrupole component while attenuating a magnetic hexapole component in the region where neutral atoms are captured, so that neutral atoms can be effectively captured, and which reduces an applied current and/or an external magnetic field by generating the magnetic field, thereby enabling miniaturization of the whole device.
2. Description of the Related Art
Magneto-optical trap [MOT] is a publicly-known technology in the field of atom optics. By using a magneto-optical trap, neutral atoms can be captured by irradiating laser beams of well-adjusted oscillating frequency along the axis of symmetry of the magnetic field lines in a quadrupole magnetic field. Since a magneto-optical trap can capture neutral atoms in the central region thereof and simultaneously perform the laser cooling, it is used as a method of cooling for most of the experiments in the field of atom optics including the Bose-Einstein condensate-generation experiment.
Generally, a quadrupole magnetic field in a magneto-optical trap is generated using anti-Helmholtz coils which are formed by placing a pair of circular coils opposite each other. However, as will be described below, a quadrupole magnetic field can also be generated by superposing a magnetic field, which is generated by an electric current flowing through a single linear wire, on a uniform bias magnetic field.
FIGS. 1(a) and 1(b) are conceptual diagrams showing relationships between an electric current and a magnetic field. FIG. 1(a) is a drawing showing a condition of a magnetic field when an infinite linear current (I) flows along an electrode (1) on the z-axis, while FIG. 1(b) is a drawing showing a condition of a magnetic field when a uniform bias magnetic field is further applied in the positive (+) direction on the x-axis. As shown in FIG. 1(a), when the infinitely long linear current (I) flows along the electrode (1) on the z-axis, a concentric circular magnetic field is generated around the linear current (I) due to Ampere's law. The magnetic flux density of the magnetic field thus generated is indicated by the following equation (i):
                                          B            x                    =                                    -                                                                    μ                    0                                    ⁢                  I                                                  2                  ⁢                  π                                                      ⁢                          y                                                x                  2                                +                                  y                  2                                                                    ,                                  ⁢                              B            y                    =                                                                      μ                  0                                ⁢                I                                            2                ⁢                π                                      ⁢                          x                                                x                  2                                +                                  y                  2                                                                    ,                                  ⁢                              B            z                    =          0                                    (        i        )            
In the equation (i), μo indicates the magnetic permeability of vacuum. By adding a uniform bias magnetic field Box in the positive (+) direction on the x-axis, the flux density is indicated by the following equation (ii):
                                          B            x                    =                                                    -                                                                            μ                      0                                        ⁢                    I                                                        2                    ⁢                    π                                                              ⁢                              y                                                      x                    2                                    +                                      y                    2                                                                        +                          B                              0                ⁢                x                                                    ,                                  ⁢                              B            y                    =                                                                      μ                  0                                ⁢                I                                            2                ⁢                π                                      ⁢                          x                                                x                  2                                +                                  y                  2                                                                    ,                              B            z                    =          0                                    (        ii        )            
It is seen from the equation (ii) that the zero-point of the magnetic field is formed at the point (0, μ0I/(2π B0x), 0) on the y-axis. The zero-point is represented by ‘Q’. The distributed magnetic field lines in this condition are schematically illustrated in FIG. 1(b). As seen in FIG. 1(b), the zero-point Q of the magnetic field forms a quadrupole magnetic field. Neutral atoms can be captured at the zero-point Q on the quadrupole magnetic field and the laser cooling can be performed.
In fact, neutral atoms can be captured three dimensionally by further adding a quadrupole magnetic field in the z-direction. FIGS. 2(a)-2(c) are conceptual diagrams showing configurations for adding a quadrupole magnetic field in the z-direction and the conditions of the magnetic field. FIG. 2(a) shows an example in which a magnetic field is impressed from outside using, for instance, anti-Helmholtz coils in the z-direction; FIG. 2(b) shows an example in which a path of electric current is deformed to a U-shape by forming both ends of the coils into a U-shape; and FIG. 2(c) shows an example in which a path of electric current is deformed to a Z-shape by forming both ends of the coils into a Z-shape. In other words, while it is preferable that a required magnetic field is added from outside by using, for instance, anti-Helmholtz coils in the z-direction as shown in FIG. 2(a), in order to generate a required magnetic field in the z-direction, since a required magnetic field in the z-direction can be provided from the arm portions parallel to the x-axis, by modifying both ends of the conductor where electric current flows to a U-shape as shown in FIG. 2(b), this type is used more frequently than that of FIG. 2(a) (See e.g. non-patent document 1 below). However, in the method shown in FIG. 2(b), since the arm portions parallel to the x-axis also generate a bias magnetic field in the y-direction, a bias magnetic field in the y-direction has to be newly added externally in order for the compensation.
In addition, in order for a general magneto-optical trap to cool the atoms three dimensionally, laser lights are irradiated from six directions along the axis of symmetry on a magnetic field towards the central portion of the trap which is composed of a quadrupole magnetic field generated by anti-Helmholtz coils, etc. It is known, however, that magneto-optical traps composed of a linear current and a bias magnetic field include one in which a total reflection mirror for the laser lights is placed on the x-z plane, and requiring merely four leaser lights instead of six as required originally by reusing the leaser lights reflected by the total reflection mirror. Such a magneto-optical trap is called a “Surface magneto-optical trap”, a “Mirror magneto-optical trap”, or a “Mirror MOT”, and is often used as a compact magneto-optical trap. It is to be noted that the method in which six laser lights are directly irradiated to a proximity of a conductor without using a mirror is called a “Wire trap”.
A modified configuration of a linear current to a Z-shape as in FIG. 2(c) can generate a magnetic field with a curvature in the z-axis direction from the two arms parallel to the x-axis. Herein, “a magnetic field with a curvature in the z-axis direction” indicates a space-dependent magnetic field in which the magnitude of the magnetic field B on the z-axis is proportional to z2. The magnetic field thus obtained can steadily capture neutral atoms since a confinement potential in the z-axis direction becomes a harmonic type being proportional to z2. However, a magneto-optical trap cannot be composed on this magnetic field since the z component of the magnetic field faces the positive (+) direction of the z-axis everywhere in the magnetic field. Namely, a configuration with the magnetic field as described in FIG. 2(c) is used as a “magnetic trap” which can only capture neutral atoms without using the laser light. Generally, a magnetic trap composed by superposing a quadrupole magnetic field on the x-y plane and a magnetic field with a curvature in the z-axis direction is called a “Ioffe-Pritchard type magnetic trap”, and a rod-shaped conductor which is provided in parallel to the x-y plane is called a “Ioffe bar”. A magnetic trap, along with a magneto-optical trap, is an indispensable device in the field of atom optics research.
Three different types of the surface magneto-optical traps and the surface magnetic trap indicated in FIGS. 2(a)-2(c) have a commonality that they generate a quadrupole magnetic field on the x-y plane by superposing a magnetic field with a narrow linear conductor for providing electric current onto a uniform bias magnetic field form outside; and since they can capture atoms in the extreme vicinity of a plane substrate, their application possibilities, such as in an atom interferometer, a quantum gate, and the like have been attracting attention, and researches have been actively performed.
Meanwhile, as indicated in FIG. 1(b), a quadrupole magnetic field generated by superposing a magnetic field generated by a single narrow linear conductor onto a uniform external bias magnetic field becomes considerably asymmetrical as distanced away from the center of the trap, thereby deviating from an ideal quadrupole magnetic field. If a magneto-optical trap is composed by using such a magnetic field, there is a problem that the effective capacity in the space where atoms drifting in the vacuum are captured becomes limited, so that sufficient numbers of atoms cannot be captured.
FIG. 3 is a conceptual diagram showing the condition of a three dimensional magnetic field wherein the width of a linear conductor in FIG. 2(b) is enlarged in the x-direction. As shown in FIG. 3, when the width of the linear conductor shown in FIG. 2(b) is enlarged in the x-direction, the uniformity of the magnetic field around the conductor increases, so that the far-field magnetic profile is improved and the quadrupole reaches further away. As a result, an effective capacity where atoms can be captured is enlarged, so that a greater number of atoms can be captured (see non-patent document 2 below). However, even when such a plate conductor is used, a magnetic distortion cannot be completely compensated, and an extra electric current has to be flowed through in proportion to the widened portion of the conductor. Therefore, the amount of heat generated from a conductor part is increased. Since a magneto-optical trap and a magnetic trap are placed in an ultrahigh vacuum device, there may be a situation where gas is emitted from a surface of a conductor when the amount of heat generated is increased, which is not desirable.
Also, a configuration with a Z-shaped conductor as shown in FIG. 2(c) is used for composing a magnetic trap which does not use the laser light, so that a magnetic field does not have to be strictly uniformed. However, a relatively large bias magnetic field has to be applied in order to capture atoms reliably. Furthermore, since the two Ioffe bars extending in the x-direction from both ends of the central conductor are relatively long, an unnecessarily large z-directed bias magnetic field is generated, so that in order to compensate said bias magnetic field, a large z-directed bias magnetic field has to be further added from outside. Accordingly, there is a problem that the whole device cannot be miniaturized even when a magnetic trap with a Z-shaped conductor show in FIG. 2(c) is used.
[Non-patent document 1] J. Reichel, W. Hänsel and T. W. Hänsch, “Atomic micromanipulation with magnetic surface traps,” Phys. Rev. Lett. 83, 3398 (1999).
[Non-patent document 2] S. Wildermuth, P. Krüger, C. Becker, M. Brajdic, S. Haupt, A. Kasper, R. Folman and J. Schmiedmayer, “Optimized magneto-optical trap for experiments with ultracold atoms near surfaces,” Phys. Rev. A 69, 030901(R) (2004).